STANNIC CHLORIDE 109 1. Impurities in general.—When the aqueous solution is treated with hydrogen sulphide and filtered, the filtrate should leave no appreciable residue on evaporation. 2. Free Chlorine.—The i : 10 solution is treated with iodide-starch paste. 3. Sulphuric Acid.—The I : 10 solution is tested with barium chloride, the barium sulphate being weighed if necessary. 4. Nitric Acid.—The concentrated solution is tested with sulphuric acid and ferrous sulphate. 5. Stannous Chloride.—The i : 10 solution is tested with mercuric chloride (white or grey precipitate). Quantitative determination as under " Stannous Chloride." 6. Stannic Oxide (Metastannic Acid).—This is indicated by an insoluble white deposit in the liquid chlorides. The solid chloride (1-2 grams) is treated with excess of sodium hydroxide, which will dissolve it completely in absence of the oxide. 7. Ammonia.—The i : 10 solution is boiled with excess of sodium hydroxide. 8. Lead.—The precipitate formed by hydrogen sulphide (test i) is treated with yellow ammonium sulphide : in presence of lead an insoluble black residue remains. 9. Iron.—The i : 10 solution, acidified with HC1, is tested with a few drops of potassium thiocyanate (red coloration). 10. Alkali Salts (Sodium Chloride).—These are detected by test i. For quantitative determination, i gram of substance is dissolved in about 500 c.c. of water and the liquid boiled to complete decomposition of the stannic chloride into the oxide and filtered, the precipitate being washed and the total filtrate evaporated to dryness, dried at 120° and weighed.1 11. Determination of the Tin and Hydrochloric Acid.—i gram of substance is boiled with 500 c.c. of water as in test lo.1 The insoluble part is collected on a filter, washed, dried, ignited and weighed as Sn02. i part SnOa =078808 part of Sn. The filtrate is titrated with N-alkali in presence of phenolphthalein. i c.c. N-alkali = 0-0365 gram HC1 or 0-0355 gram Cl. * * * Stannic chloride of good quality should contain only small proportions of the above impurities. The content of SO3 should not exceed 0-04%, and iron should be only in traces. The solid chloride should contain not less than 45-4% Sn. Sodium chloride often occurs to the extent of 5% or more. The so-called Tin Compounds or Solutions, Tin mordants, Nitvomuviates or SulpJiomunates of Tin consist of solutions of stannic and stannous chlorides with varying proportions of sulphuric and nitric acids, ammonium, 'zinc or iron salts, sodium chloride, etc. ; their value depends mainly on the proportion of total tin present. 1 If stannous salts are present, a little bromine water is added prior to the boiling. in 50 c.c. of water.